Chem Film (Chromate Conversion)
Chemical conversion coating (MIL-DTL-5541 Type I/II) for aluminum. Provides corrosion protection, maintains electrical conductivity, and serves as a primer for paint adhesion.
About Chem Film (Chromate Conversion)
Chem film, also known as chromate conversion coating or Alodine (a common trade name), is a chemical surface treatment for aluminum that creates a thin, protective chromate layer through a controlled chemical reaction. Unlike anodizing, which builds a thick oxide layer, chem film produces an ultra-thin coating (typically 0.025-0.1 µm) that provides excellent corrosion resistance while maintaining the electrical conductivity of the aluminum surface. This unique combination makes it indispensable for aerospace and electronics applications.
Chem film is specified per MIL-DTL-5541, the military standard that defines two classes: Type I (hexavalent chromium, gold/iridescent color) and Type II (trivalent chromium or non-chromate, clear/colorless). Type I has historically been the aerospace standard due to its superior self-healing corrosion resistance, but environmental regulations are driving adoption of Type II (RoHS-compliant) alternatives that approach comparable performance. Both types serve as excellent paint primers, significantly improving the adhesion and durability of subsequent paint or powder coat layers.
The chem film process is fast, economical, and adds essentially zero dimensional buildup, making it suitable for precision components, electrical connectors, RF enclosures, and grounding surfaces. It is commonly specified for aluminum components in aircraft structures, satellite hardware, military equipment, telecommunications enclosures, and medical instruments. For parts requiring both corrosion protection and electrical grounding, chem film is often the only viable surface treatment option.
Finish Properties
Available Types
Type I (Hexavalent Chromium)
The traditional aerospace-grade chromate conversion using hexavalent chromium compounds. Produces a distinctive gold/iridescent appearance. Offers superior self-healing corrosion resistance—minor scratches reseal themselves as chromium ions migrate to the damaged area. Specified extensively in legacy aerospace programs per MIL-DTL-5541 Class 1A. Being phased out in new designs due to hexavalent chromium environmental and health concerns.
0.025-0.1 µm
Type II (Trivalent Chromium / Non-Chromate)
RoHS and REACH-compliant alternative using trivalent chromium or non-chromate chemistries. Produces a clear to slightly bluish-iridescent appearance. Corrosion resistance approaches Type I performance in newer formulations. Preferred for new designs, commercial products, and applications subject to environmental compliance requirements. Becoming the industry standard as performance continues to improve.
0.025-0.05 µm
How It Works
The chem film process is a simple immersion or spray application performed at room temperature. Aluminum parts are first cleaned and deoxidized in an acid bath to remove surface oxides and contaminants, ensuring a uniformly reactive surface. The parts are then immersed in or sprayed with a chromate solution (hexavalent or trivalent chromium compounds in acidic media) for 1-5 minutes. The solution reacts with the aluminum surface, dissolving a small amount of aluminum and depositing a complex chromium oxide/hydroxide gel layer in its place. After the prescribed contact time, parts are rinsed in clean water and air-dried. The resulting coating continues to cure and harden over 24 hours. No heat treatment or electrical current is required, making the process fast and suitable for large batches.
Design Guidelines for Chem Film (Chromate Conversion)
Specify Class 1A (thicker coating) for maximum corrosion protection or Class 3 (thinner coating) when maintaining low electrical contact resistance is the priority.
Do not apply chem film to surfaces that will be subsequently anodized—the chromate layer prevents proper anodic oxide formation and must be stripped first.
For mixed-finish parts requiring both chem film (on grounding surfaces) and anodize (on other surfaces), anodize first, then selectively apply chem film to masked-off areas.
Be aware that chem film provides minimal abrasion resistance—it is not a wear coating and will be rubbed away by repeated mechanical contact. Use it on protected or static-contact surfaces.
Compatible Materials
Materials that can be finished with chem film (chromate conversion).
Chem Film (Chromate Conversion) FAQ
Chem film produces an ultra-thin (0.025-0.1 µm) chromate layer that maintains electrical conductivity, while anodizing produces a much thicker (5-70 µm) aluminum oxide layer that is electrically insulating. Chem film is preferred when electrical grounding or conductivity is required, and anodizing is preferred when maximum corrosion resistance, wear resistance, or color finishing is needed. Chem film is also faster and less expensive than anodizing but provides less mechanical protection.
Type I hexavalent chromium chem film is still allowed and widely used in aerospace and defense applications under ITAR and military specifications. However, it is restricted under RoHS and REACH regulations for commercial products sold in the EU and many other jurisdictions. New designs should specify Type II (trivalent chromium) alternatives unless legacy aerospace specifications mandate Type I. The industry is actively transitioning to Type II formulations.
Chromate conversion coatings can also be applied to zinc, cadmium, magnesium, and zinc-plated steel surfaces using different formulations. However, the term "chem film" and MIL-DTL-5541 specifically refer to aluminum chromate conversion. For zinc-plated steel, chromate passivation per ASTM B633 is the equivalent process. Each metal requires a formulation specific to its surface chemistry.
No. Chem film adds an immeasurably thin layer (0.025-0.1 µm, less than 1/10th of a micron per side) that has zero practical impact on part dimensions. This makes it ideal for precision components, threaded fasteners, and tight-tolerance assemblies where even small dimensional changes are unacceptable. No masking is required for dimensional reasons, though masking may be used to keep specific surfaces bare for bonding or welding.
Yes, chem film is one of the best paint primers for aluminum. The chromate layer provides excellent adhesion for epoxy primers, polyurethane topcoats, and powder coatings. MIL-DTL-5541 Class 3 (thinner coating) is typically specified as a paint base because it provides good adhesion without excessive chromate that could interfere with paint bonding. In aerospace applications, chem film plus epoxy primer is the standard corrosion protection system for painted aluminum structures.
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